My earlier geared Inbred frame has a chainstay bridge and will run disc or V rear. I’m running V and have fitted a rack with minimal faff.
What you get with a touring frame (eg LHT or Thorn Sherpa) should be plenty of braze ons and long chainstays (460mm on LHT vs 425mm on Inbred). This means that more of the rear luggage weight can be forward of the rear axle and still have heel / pannier clearance. Having the weight within the wheelbase helps with stability and reduces ‘tail wagging dog’ effect. The tubing is also chosen to be better with a load. People, especially lighter ones, sometimes find touring frames ‘unresponsive’ and better with a load than without.
You could try moving wheels as far back as they’ll go in Roadrat dropout to lengthen chainstay and see if that helps. I suspect though that what makes it a nice responsive ride unladen is what makes it noodly laden.
Have you had a chance to try your roadrat with the luggage aboard, but without the followme attached. I’ve not tried one myself, but I don’t like the feel of a trailer bike attached unless it’s a 2 wheeled one. It might be worth considering a childback tandem as that will feel much better than any attachment arrangement. There was one on ebay recently which went for £360 with S&S couplings. Tandems are much more difficult to transport by train than solos if you need to do that though.
Might be worth swapping and seeing how MrM’s Inbred feels with the followme attached and also your Roadrat with the child trailer. You’d get some idea of whether swapping to an Inbred frame would be much better for you.
I think if you ride a 54cm or smaller frame a 26″ LHT will come up sometime. It will take longer if you want a larger frame as they’ve only recently started doing them with 26″ wheels.